Auditlog is a poorly written overlay that hopefully will be deleted
in the future. It kills perf, and in general is not recommended for
use. If you want a record of changes, I suggest you look at the
accesslog overlay.

1. Can we use an LDIF backend with the auditlog overlay?

Why are you stuck on auditlog? As I said, don't use it.

2. Has anyone written a program to convert the auditlog data to an
LDIF format that can be applied with ldapmodify?

No idea. As I said, it's not really a usable piece of software.

One of the advantages of syncrepl (Although I use and prefer
delta-syncrepl, since syncrepl has not proven reliable in my
experience)

We are avoiding delta-syncrepl because, for reliability, we want to
use mirror-mode, and at
http://www.openldap.org/doc/admin24/replication.html#MirrorMode%20replica
tion
under section 18.2.3.2. Arguments against MirrorMode is written:
"Delta-Syncrepl is not yet supported"
3. Does this mean that we are aiming for a less reliable replication
with syncrepl that with delta-syncrepl?

You can still enable the accesslog overlay regardless of what replication
mechanism is used between the mirrors. As for the reliability, as I said,
in my experience, delta-syncrepl has been reliable.

4. Is there any prospect of delta-syncrepl being supported with
mirror-mode?